This invention relates to an electronic security device for use, for example, as a door lock.
Various forms of electronic security device are already known, in which a suitably coded key is recognised by an electronic circuit to permit operation of a door bolt. When such a device is used, in a situation where it is desirable regularly to change the code to which the electronic circuit will respond, it becomes necessary to ensure that the circuit and the key currently in use have the same code. This can be achieved by connecting all the electronic circuits to a common control centre, but this is clearly disadvantageous when it is desired to convert the locks of an existing system to electronic locks.
To overcome this problem several solutions have been suggested. One such suggestion is to store in the circuit a fixed sequence of codes and to use each new key to call up the next code of the sequence. Another suggestion utilizes keys each of which has two codes on it, namely the code currently in use and the next code to be used. This next code is stored in the electronic circuit until a new key is used when it is replaced by a `new` next code.
None of the previously suggested systems provides the ideal solution to the problem. At any given time the "next" code is already established and this casts some doubt on the security of the system.
In accordance with the present invention there is provided an electronic security device including memory device for storing a combination code, a key reading device for reading from a key device data representing both the combination code and calculation data, and an electronic circuit connected to said memory device and to said key reading device and serving to produce an output for releasing a security device when the combination code from the key reader device matches that in the memory and also serving to change the content of the memory to match the combination code from the key reading device when the content of the memory matches a new combination code calculated by the electronic circuit utilizing the existing content of the memory and the calculation data.
The invention is particularly (but not exclusively) applicable to hotel door lock systems and it will readily be appreciated that in such an arrangement it gives many advantages over previously proposed systems. In particular each new combination code can be randomly selected--the appropriate calculation data being worked out at the central key issuing station. Each key contains no information relating to previous or future combination codes and this makes it extremely difficult for a would-be thief to analyze the system and forge a key.